https://www.minecraftskins.com/skin-editor/ is an editor I found very helpful. of course, you can get a lot more editing possibilities with software like paint.net or photoshop, but minecraftskins is a great starter tool as it gives a user-friendly interface and very handy tools. it also has a collection of user-made skins that you can use for reference. the best feature is that it allows you to see and paint your skin in 3D.
my personal tip is to utilize the outer "jacket" layer. you can give details of your skin more texture and appear less flat. also, be sure to use subtle but different colors for each part of the skin you want to add (such as shading a jacket or skin tones).
by default, Minecraft puts it's directory in your C: drive, or Local Disk. if you want, you can set it to whatever drive or folder that has enough space. say, if you had a network of drives and you have a drive dedicated for game files, you can tell the launcher to put the .minecraft files there instead of filling up your local disk.